The invention relates to a process for the control of turbo compressors with regard to the avoidance of surging and to a device for the execution of this process.
In compressors, surging is the pulsating or periodical reflux of a pumping medium from the delivery side to the suction side. This situation arises e.g. when the delivery pressure or the delivery pressure to suction pressure ratio is too high and/or the volume rate of flow is too low. Therefore, a surge limit line can be defined univocally in the performance graph (a graph of the relationship of turbo compressor parameters) which divides the performance graph into a stable and an instable section. As a rule, the surge limit line is curved, i.e. its ascending gradient changes within the performance graph. In a frequently used characteristics diagram, wherein the coordinates are volume rate of flow and pressure, e.g., the surge limit curve flattens as the pressure increases. For other performance graphs featuring guide blade position, rpm, head of the compressor etc. the same applies.
In order to prevent compressors from surging, a blow-off curve, or anti-surge control curve, may be defined on the performance graph at a safety distance from and parallel to the surge limit curve. When the actual working point approaches the blow-off curve, a blow-off valve or by-pass valve is opened to a small extent or a great extent depending upon the system. Due to this the actual value of a controlled variable, in particular the value of the volume rate of flow, does not surmount the set value determined by the blow-off curve and the command variable, in particular the delivery pressure. The anti-surge control can be considered a flow control with a variable flow set point, which is determined from the actual measured compressor discharge pressure (or pressure ratio if this is plotted on the vertical axis). The anti-surge controller output is changed to such an extent that the flow to the process plus the flow through the blow-off or by-pass valve is identical to the flow at the blow-off line at the respective actual discharge pressure. If process flow decreases, controller output changes to open the blow-off valve, so that the set point value, i.e. flow at the blow-off line, and the actual value match. If the blow-off curve has a constant gradient, the relationship between pressure changes and set point changes is constant. If the blow-off curve has a gradient which is not ccnstant, the set point changes are small where the blow-off curve is steep, and the set point changes are large where the blow-off curve gradient is less steep. Therefore, the gain or amplification of the system is not constant.
There are also automatic controls wherein the volume rate of flow serves as the command variable for the determination of the set value and wherein the delivery pressure is the controlled variable to be adjusted with regard to the set value.
The curved course of the blow-off curve results in a preset change of the command variable, or controller output, (e.g. discharge pressure) at various points on the blow-off curve which results in changes in the set value for the controlled variable. This results in variations in the control loop amplifications.
Anti-surge limit control means are safety control means and are normally activated to work near the stability limit in order to guarantee the best compressor protection possible. The stability limit is the limit where the closed control loop becomes unstable due to a too-high closed loop gain or amplification. The position of the stability limit is heavily influenced by the overall amplification of the control loop. A high degree of overall amplification is most likely to lead to instability cf the system. However, to get the best control results, the gain should be selected as high as possible. The higher the gain, the better the control result.
In order to achieve the most constant overall amplification possible, it is suggested in compliance with the EP-A-O 223 208 (see also U.S. Pat. No. 4,789,298) to compensate the influence of the valving line on the amplification factor by taking into account the ascending gradient of the valving line in various characteristic sections. However, it has turned out that a constant overall amplification can be achieved only within limits as the non-linear gradient cf the compressor's characteristic curve also creates a change in the sectional amplification and thus influences the overall amplification.